Among currently employed processes for synthesizing acetic acid, one of the most useful commercially is the catalyzed carbonylation of methanol with carbon monoxide as set forth in U.S. Pat. No. 3,769,329issued Oct. 30, 1973. The carbonylation catalyst comprises rhodium along with a catalyst promoter exemplified by methyl iodide. The reaction is conducted with the catalyst being dissolved in a liquid reaction medium through which carbon monoxide gas is continuously bubbled. The rhodium-catalyzed carbonylation of methanol to acetic acid as described in this patent is very selective to the acetic acid product and as such offers substantial advantages over oxidation processes in which multiple distillations are required to separate the acid product from the other oxygenated products which are formed.
An improvement in the rhodium-catalyzed carbonylation of methanol to acetic acid is set forth in commonly assigned U.S. Pat. Nos. 5,001,259; 5,026,908 and 5,144,068. As disclosed therein, acetic acid is produced by the carbonylation of methanol in a reaction medium comprised of methyl acetate, methyl iodide, rhodium and an additional iodide salt such as lithium iodide and wherein the water concentration in the reactor is maintained at below 14 weight percent. This process has been characterized as the "low water" carbonylation process as distinguished from prior commercial adaptation of the teachings in U.S. Pat. No. 3,769,328 wherein greater than 14 wt. % water was maintained in the reaction medium and which prior process has been characterized as the "high water" carbonylation process. It has been found that catalyst stability and the productivity of the carbonylation reactor can be maintained at surprisingly high levels even at the low water concentration of below 14 weight percent and even at very low water concentrations of 4 weight percent or less. The high reaction rates and improved productivity are quite surprising in view of the prior art which disclosed the benefits of high water to improve reaction rates and in view of the high propensity of the rhodium to precipitate from the solution especially at water levels of below 14 weight percent. By utilizing less water in the reaction medium, downstream purification to remove water from the acetic acid product is substantially reduced. Accordingly, by providing increased productivity in the reactor, the overall productivity of the low water carbonylation process is vastly improved over productivity in the high water process.
The reaction system which is employed in the rhodium-catalyzed carbonylation of methanol to acetic acid comprises a liquid-phase carbonylation reactor, a flasher for catalyst recycle, a methyl iodide-acetic acid splitter column and a drying column. The carbonylation reactor is typically a stirred autoclave within which the reacting liquid contents are maintained automatically at a constant level. Into the reactor there are continuously introduced fresh methanol, sufficient water, recycled catalyst solution from the flasher base and recycled methyl iodide, methyl acetate and water from distillation. The reactor product is discharged to the flasher wherein a liquid catalyst solution is withdrawn as a base stream containing predominately rhodium and acetic acid as well as the iodide salt along with lesser quantities of methyl acetate, methyl iodide and water while the overhead of the flasher is a vapor comprising largely the acetic acid product along with methyl iodide, methyl acetate and water. The catalyst solution withdrawn from the flasher base is recycled to the reactor. The flasher vapor is then fed to the distillation unit wherein the product acetic acid is typically withdrawn from the middle of the splitter column and then directed to the drying column for removal of water. The overhead from the splitter column contains a heavy phase of aqueous methyl iodide and methyl acetate and a light phase comprising aqueous acetic acid. The water removed from acetic acid product in the drying stage contains small amounts of acetic acid and can be combined with the light aqueous acetic acid phase from the splitter column. The heavy phase, light phase and water from the distillations are returned to the reactor.
Implementation of the low water carbonylation process necessitated changes in the means to control the liquid levels in the reactor and flasher that were used under high water carbonylation conditions. Unfortunately, such changes have led to a wider variability in these liquid levels as well as reactor product flow rate and flasher recycle flow rate per a given methanol feed rate to the reactor. While a steady state of dry acetic acid product has been maintained from the distillation system per methanol feed rate, process inefficiency in the form of variable liquid levels and flow rates of the intermediate process streams is economically disadvantageous.
Prior to the introduction of the low water operating conditions, the exothermic heat of reaction could be absorbed by the solvent medium including water and removed almost solely by the reactor product flow to the flasher. Thus, to control temperature in the reactor there was established a fixed relationship between the methanol feed rate and the reactor product flow rate to the flasher. This fixed relationship led to narrow variability in the liquid level in the reactor and intermediate process streams. However, upon implementation of the low water technology as described in the aforementioned commonly assigned U.S. Pat. No. 5,001,259, the amount of water in the reactor was reduced and, as a consequence of the reduced heat sink, the reactor and the catalyst recycle flow from the flasher base were provided with coolers to remove the heat of reaction in addition to heat removal via the product flow to the flasher. With the addition of the cooling means, the fixed relationship between the methanol feed rate and the product flow rate from the reactor to the flasher in order to control temperature no longer existed. Accordingly, the use of the coolers and the consequent loss of the fixed relationship between methanol feed and the manipulated reactor product flow rate to the flasher resulted in a wider variability of the reactor and flasher liquid levels as well as the reactor product and catalyst recycle flow rates even under a constant methanol feed rate to the reactor.
Prior to the present invention, liquid level control in the reactor and flasher was achieved by level controllers with both proportional and integral control elements. The level controllers affected changes in reactor product and catalyst recycle flow rates in response to deviations in the liquid levels so as to maintain the desired set liquid levels in the respective vessels. The loss of the fixed relationship between methanol feed rate and product flow rate to the flasher as described above resulted in a greater reliance on the integral controller to maintain the proper liquid levels and flow rates. The integral control mode, however, continuously seeks to achieve zero off-set from the set point of the controller and, accordingly, substantial oscillation occurred in the affected variables during the corrections by the controller. In particular, substantial variation occurred in the reactor level and reactor product flow rate, flasher level and catalyst recycle from the flasher to the reactor as well as recycle flow rates from the distillation apparatus including the splitter column and drying column as the integral controller attempted to achieve zero off-set from the set liquid levels in the respective vessels. Importantly, in addition to changes made in the reactor product flow rate and catalyst recycle flow rate in response to deviations of the liquid levels from the level set points of the reactor and flasher, changes in the respective flow rates were also made proportional to changes in the methanol feed rate by employment of a multiplier which commanded the set point for the flow controllers. In fact, a major reason for liquid level deviations from the set point was due to changes in the methanol feed rate to the reactor. Unfortunately, the proportional changes made in the reactor product flow rate and catalyst recycle flow rate relative to the changes in methanol feed rate did not accurately represent the changes empirically taking place during the carbonylation reaction inasmuch as the reactor product flow rate and catalyst recycle flow rate do not change proportionally to changes in the methanol feed rate at steady state.
A proportional only control would be insufficient to correct the deviations of the liquid level as a substantial off-set from the set point of the controllers would likely still remain if coupled with the inaccurate multiplier to adjust for changes in the methanol feed rate. The integral controller while correcting the liquid level off-set causes substantial oscillations in the process variables during the operation thereof as described above. The wide variability in liquid levels and flow rates of the intermediate process streams as a result of the integral controller and inaccurate multiplier leads to process inefficiencies. The inefficiencies are manifested in excessive liquid being treated in the distillation train, excessive steam requirements for distillation, poor product quality and eventually lower methanol feed rates to reduce purification needs and consequent reduction in productivity.
Accordingly, there is a need to reduce the variability of the liquid levels in the reactor and flasher as well as the reactor product flow rate to the flasher and catalyst recycle flow rate from the base of the flasher to the reactor relative to both a constant and changing methanol feed rate to the reactor. The objects of the present invention address such need. Thus, the objective of the present invention is primarily directed to improve the operation of a reactor-flasher combination used in the rhodium-catalyzed carbonylation of methanol to acetic acid but can be directed to reduce operational variability in any reaction using the reactor-flasher combination.